... I think this is because of its limited graphics capabilities.
The Mini's graphics capabilities aren't really limited. Every graphics card in the last 10+ years has had such good 2-D performance (i.e., like what you're doing with Photoshop) that it's so irrelevant that the benchmarking web sites don't even test it anymore. The HD 4000 in a Mini will be just as good for your Photoshop work as any other graphics card.
Are you using an SSD? That might give you a pretty huge performance boost.
Another thing is that recent versions of Photoshop might have gotten slower. I used to use a version of Photoshop that was > 5 years old, and when I updated to the latest one I noticed a pretty big slowdown. Bloat, I guess.
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Let's face it: the Mini is not designed to be a pro machine. It's amazing how much it has evolved and how capable it is... and how little it costs. But if you rely on professional apps and push them hard then the iMac, MacBookPro and MacPro are what you should be using.
Even if Apple retained the discrete GPU option the Mini would be relatively anemic compared to its beefier brethren based on Apple's past designs. It's too bad the company won't offer a so-called "headless iMac" for those who need more than a Mini can offer and who don't want an all-in-one computer or a top of-the-line tower Mac.
You have to realize that a discrete graphics chip is only valuable if you're doing 3-D work (usually 3-D gaming since 3-D raytracing will use the CPU, not GPU) or if you're doing something that's OpenCL accelerated, which basically means a small number of video filters and certain scientific computing tasks.
Otherwise GPU performance is basically irrelevant, and the Mini will be just as fast at almost everything as a MacBook Pro or a Mac Pro from 2010 or earlier. You have to step up to an 8 or 12 core Mac Pro to get something that's significantly faster than the 2.6 Mini.
I'm not trying to say the Mini is the best thing you can buy, but to give things a little perspective, the high end Mini today is faster than basically any Mac you could have bought 2-3 years ago, and people were using Macs 2-3 years ago just fine to do HD video editing, high end photography, etc. etc.
